1. Field of the Invention
The present invention relates to a medical drilling machine with a drive shaft which is rotatably held and is housed in an X-ray transparent casing, and more particularly to a technique for achieving accurate boring operation.
2. Description of the Related Art
In surgical operation or the like, there are instances where a hole must be bored with a medical drilling machine at a position that is not directly observable inside the body of a patient. Typically, such holes are bored in bone or transplant materials transplanted into the body. When boring the holes at a position which is not directly observable, X-ray images must be used to position the drill bit accurately.
FIG. 12 shows a conventional medical drilling machine disclosed in Japanese Patent Laid-Open Publication No. Hei 9-19437.
This medical drilling machine 1 includes: a casing 11 made of X-ray transparent material; a drive shaft 16 housed inside the casing; and a pair of bevel gears 12, 13 for transferring rotational force to the drive shaft 16. One of the bevel gears 12 is coaxially arranged along the same axis as the drive shaft 16, and the other bevel gear 13 is provided at one end of an input shaft 15. The input shaft 15 is held in the casing 11 by a bearing 14. The drive shaft 16 and the bevel gears 12, 13 are also made of X-ray transparent material.
The drive shaft 16 is held rotatably by two similarly shaped bearings 17, 18 made of X-ray opaque materials. A chuck 20 is provided at one end of the drive shaft 16, and a drill bit 19 is gripped by the chuck 20.
Next, a simple explanation is given regarding a procedure using the medical drilling machine 1 to form a bolt hole in a bone of a human body.
Before using the medical drilling machine 1, first, a drive motor 21 is attached to the input shaft 15. Then, X-rays are radiated along the direction in which the bolt hole will be bored (hereinafter, referred to as the “ideal axial line R”). The doctor firmly clutches the medical drilling machine 1 in his or her arm, and when the drilling machine enters into an X-ray radiation field T, an image such as shown in FIG. 13 is obtained by the X-rays. An axial tip 19a of the drill bit 19, an end surface 18a of the bearing 18, and the human bone B appear in the X-ray image. Therefore, the bolt hole can be formed in the bone by making the drill bit 19 rotate as the medical drilling machine 1 is advanced forward in a straight direction.
However, it is difficult actually to form an ideal bolt hole. The reason for this is that when the drill bit 19 is advanced forward in the direction of the ideal axial line R, there inevitably occurs misalignment of the axial center, or slanting of the drill bit 19, or other similar problems, and prompt correction thereof was difficult.
In more specific terms, when the drill bit 19 slants with respect to the direction of the ideal axial line R, the axial centers of the two bearings 17, 18 become misaligned, causing an oval ring D to be included in the X-ray image as shown in FIG. 14.
When the doctor sees this image, the doctor understands that the drill bit 19 has slanted along a long axis D1 across the length of the oval ring D. However, there still is a problem in that the doctor can not judge which direction the drill bit 19 is slanting along the long axis D1. As a result, the doctor would adjust the direction of the drill bit 19 in the wrong direction, causing the drill hole C to slant even further, or requiring time to adjust the direction of the drill bit 19, thus creating a problem of lengthening the operation time.
Furthermore, since the drive motor 21 is provided as a separate unit from the medical drilling machine 1, there was a problem that it was very difficult for the doctor to hold. With this structure it was fundamentally difficult to position the drill bit 19 accurately and/or maintain the position. As described above, drill hole errors occurred easily, even when observing the misalignment of the two bearings 17, 18 in X-ray images.